In prior apparatus used for the rotary draw bending of pipe and tube, such as of use in automobile exhaust systems, heat exchangers and aircraft construction, a primary component is the bending head of the apparatus. The bending head comprises a rotary bend die, an opposing clamp die which clamps a section of the tube immediately preceding the section of the tube where the bend is to be formed, and a pressure die located directly behind the clamped section of the tube. As the tube is pulled around the rotary bend die, the pressure die moves substantially in unison with the tube while resisting the radial reaction force of the tube acting on the pressure die. Thus, the pressure die and rotary bend die cause the tube to be squeezed therebetween during the bending operation.
Many variable factors, such as the type of tube material, tube wall thickness, shape of the tube section to be formed, the radius of the bend and the like, need to be considered when tube bending with rotary draw bending machinery is carried out. However, although commercially acceptable tubes are manufactured with apparatus hereinbefore described, there is a need for pipe bending methods and apparatus which are capable of producing tubes of a consistent, desired quality. While not being bound by theory, applicant believes that although the tube and pressure die move substantially in unison, slight changes in the values of the above variables in consequence of changes in pipe diameter, wall thickness, the presence of impurities and the like, during the bending operation may cause small, but significant changes in the speed of the pressure die relative to that of the tube. Thus, by exerting a negative boost or slowing force to the tube by means of applying a force to the pressure die, the forward motion of the tube is restrained. Alternatively, by exerting a positive boost or quickening force to the pressure die during the bending operation the frictional force enhances forward movement of the outer tube. Thus, by monitoring the speed of the pressure die and the perpendicular force exerted on the tube and compensating for the effects of changes in the above parameters, an improved tube product can consistently be manufactured. Further, in addition by monitoring the force required to achieved the desired boost speed, tool wear and/or contamination on the tube material can be detected.